Plasmon-Enhanced Raman Sensing with Metal-Insulator-Metal Metasurfaces.

The finite difference time domain (FDTD) method is a grid-based, robust, and straightforward method to model the optical properties of metal nanoparticles (MNPs). Modelling accuracy and optical properties can be enhanced by increasing FDTD grid resolution; however, the resolution of the grid size is limited by the memory and computational requirements. In this paper, a 3D optimized FDTD (OFDTD) was designed and developed, which introduced new FDTD approximation terms based on the physical events occurring during the plasmonic oscillations in MNP. The proposed method not only required ~52% less memory than conventional FDTD, but also reduced the calculation requirements by ~9%. The 3D OFDTD method was used to model and obtain the extinction spectrum, localized surface plasmon resonance (LSPR) frequency, and the electric field enhancement factor (EF) for spherical silver nanoparticles (Ag NPs). The model’s predicted results were compared with traditional FDTD as well as experimental results to validate the model. The OFDTD results were found to be in excellent agreement with the experimental results. The EF accuracy was improved by 74% with respect to FDTD simulation, which helped reaching a near-unity OFDTD accuracy of ~99%. The λLSPR discrepancy reduced from 20 nm to 3 nm. The EF peak position discrepancy improved from ±5.5 nm to only ±0.5 nm.

Plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy, shell-isolated nanoparticle-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy, has witnessed substantial development over the past 20 years. These techniques can provide fingerprint information on target materials with sensitivities down to the single-molecule level and with sufficient spatial resolution to observe individual vibrational modes. PERS has thus found applications in diverse areas, ranging from bioanalysis to materials characterization. In this Technical Review, we survey the fundamental principles, advantages and limitations of using localized surface plasmon resonance to enhance the Raman signal in PERS. We discuss the issues that influence the sensitivity and interpretation of PERS results and provide an overview of state-of-the-art PERS applications in materials characterization, bioanalysis and the study of surfaces and interfaces. We also troubleshoot common experimental issues, largely based on our own experience. Finally, we conclude by examining future directions and issues to be addressed for the further development of PERS techniques. Plasmon-enhanced Raman spectroscopy (PERS) is a highly sensitive technique that can provide molecular fingerprint information. This Technical Review discusses the fundamental principles, advantages and limitations of PERS, key issues in using PERS and interpreting results, and state-of-the-art applications in materials characterization, bioanalysis and the study of surfaces.

Plasmon-enhanced Raman spectroscopy: Principles and applications

The growing need for energy storage systems requires batteries with even higher power and energy density with extended life and enhanced safety. This clearly calls for innovations in high energy density electrode materials as well as in design of robust and efficient charge transfer interfaces without electrolyte degradation or chemical side reactions. Charge transport across the solid electrode-liquid electrolyte interface (SLI) is believed to be one of the charge/discharge rate limiting steps. In lithium-ion batteries, the thermodynamic instability of the electrolyte at the SLI leads to formation of a passivation layer, commonly referred to as the solid-electrolyte interphase (SEI). Although the composition, morphology, and structure of SEI in lithium-ion batteries have been extensively studied, probing its evolution in situ at nanoscale is still challenging to a large extent. Plasmon-enhanced Raman spectroscopy (PERS) is promising to solve this challenge. PERS is the resonant oscillation of conduction electrons at the interface between negative and positive permittivity molecules stimulated by incident light. It includes surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). The prerequisite of employing PERS to probe the surface molecules is to have a highly ordered plasmonic nanostructured SPR substrate, which is extremely challenging. Large area (cm2) monolayers of gold nanoparticles (Au NPs) have long-range ordered nanogap arrays. The local electromagnetic field is extremely intense within the nanogap (<10 nm) region due to the coupling effect among adjacent nanoparticles, which allows for probing the molecules at immediate vicinity of SLI (distance from the solid surface is of 17 nm). 1 The Au NP monolayers exhibit high SERS sensitivity even down to single-molecule level (enhancement factor > 107). This allows for probing a broad range of trace electrolyte components, such as lithium hexafluorophosphate (LiPF6), fluoroethylene carbonate (FEC), ethylene carbonate (EC) and diethyl carbonate (DEC), etc. The investigation of a commercial lithium-ion battery electrolyte (LiPF6 in EC+DEC binary solvents) using SERS allows for the determination of the solvent coordination numbers, which ranges from 2 to 5, in sharp contrast to those calculated from bulk liquid electrolytes by standard confocal Raman and infra-red (IR) spectroscopy from 3 to 6. 2 SERS is promising to probe molecular fingerprints in nanoscale through SLI plane. However, it lacks the nanoscale resolution in sample plane. This makes it extremely hard for getting spatial heterogeneity of the SEI, which can be only on the order of 10 nm. Tip Enhanced Raman Spectroscopy (TERS) combines SERS with atomic force microscopy (AFM), capable of providing the chemical vibrational information and topography of the sample in the nanometer spatial resolution simultaneously. TERS analysis on cycled amorphous (a-Si) indicates that the nanometer scale SEI “islands” are unevenly distributed on the Si anode surface. Even for the same SEI “island”, the composition is different from point to point with inter-point distance smaller than 10 nm. The local chemical information studied by TERS is intrinsically different than that collected from the standard confocal Raman and IR spectroscopy. Acknowledgement This work is supported by the U.S. Department of Energy's Vehicle Technologies Office under the Silicon Electrolyte Interface Stabilization (SEISta) Consortium directed by Anthony Burrell and managed by Brian Cunningham

A metallic bowtie nanoring array is designed to gain high sensitive and reproducible substrate for surface‐enhanced Raman scattering (SERS) spectroscopy. The localized surface plasmon resonance (LSPR), the electric field enhancement factors (EFs) and the electric field distribution of the bowtie and bowtie nanoring array are numerically investigated by means of the finite‐difference time domain (FDTD) method. After the optimization of the particle size and the array period, the maximum electromagnetic field EF approaches 153, and the corresponding SERS electromagnetic enhancement factor (EMEF) reaches 5.4 × 108. This highly sensitive and reproducible substrate can be a good candidate for SERS applications. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Since 2000, there has been an explosion of activity in the field of plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). In this Review, we explore the mechanism of PERS and discuss PERS hotspots — nanoscale regions with a strongly enhanced local electromagnetic field — that allow trace-molecule detection, biomolecule analysis and surface characterization of various materials. In particular, we discuss a new generation of hotspots that are generated from hybrid structures combining PERS-active nanostructures and probe materials, which feature a strong local electromagnetic field on the surface of the probe material. Enhancement of surface Raman signals up to five orders of magnitude can be obtained from materials that are weakly SERS active or SERS inactive. We provide a detailed overview of future research directions in the field of PERS, focusing on new PERS-active nanomaterials and nanostructures and the broad application prospect for materials science and technology. Assisted by rationally designed novel plasmonic nanostructures, surface-enhanced Raman spectroscopy has presented a new generation of analytical tools (that is, tip-enhanced Raman spectroscopy and shell-isolated nanoparticle-enhanced Raman spectroscopy) with an extremely high surface sensitivity, spatial resolution and broad application for materials science and technology.

Metallic nanostructures exhibit superior catalytic performance for diverse chemical reactions and the in-depth understanding of reaction mechanisms requires versatile characterization methods. Plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and tip-enhanced Raman spectroscopy (TERS), appears as a powerful technique to characterize the Raman fingerprint information of surface species with high chemical sensitivity and spatial resolution. To expand the range of catalytic reactions studied by PERS, catalytically active metals are integrated with plasmonic metals to produce bifunctional metallic nanostructures. In this minireview, we discuss the recent advances in PERS techniques to probe the chemical reactions catalysed by bifunctional metallic nanostructures. First, we introduce different architectures of these dual-functionality nanostructures. We then highlight the recent works using PERS to investigate important catalytic reactions as well as the electronic and catalytic properties of these nanostructures. Finally, we provide some perspectives for future PERS studies in this field.

Plasmonic nanoantennas have earned strong recognition for their unique capability to confine light from free space into sub-wavelength dimensions with strong electric field (E-field) enhancement factor due to localized surface plasmon resonance (LSPR). Broad spectral tuning of LSPR from ultraviolet (UV) to near-infrared (NIR) is required for incident light wavelength and material sensitive plasmonic applications in different spectral regions. In this article, we introduced and designed a novel aluminum plasmonic platform consisting of a bowtie nanoantenna (BNA) array with metal-insulator-metal (MIM) configuration where LSPR peak position was broadband tunable from UV to NIR through geometric control of antenna parameters. Furthermore, we designed and numerically analyzed a plasmonic biosensor platform that detected concentration of glycerol in de-ionized (DI) water with a concentration in the range of 0 to 40 wt% (refractive index = 1.333 to 1.368) with a sensitivity of 497 nm/RIU (refractive index units). The designed plasmonic platform can also be used as a surface-enhanced Raman scatting (SERS) substrate with enhancement factor as high as 4.82 × 109 for 1042 nm excitation wavelength. The reported hybrid dielectric-metallic plasmonic nanostructured system is a universal plasmonic platform for a wide range of applications including single-molecule SERS, biosensing, fluorescence microscopy, plasmonic nanocavity, nanolasers, and solid-state lighting.

Ramanspectroscopy has capability for fingerprint molecular identificationwith high sensitivity if weak Raman scattering signal can be enhancedby several orders of magnitudes. Herein, we report a heterostructure-basedsurface-enhanced Raman spectroscopy (SERS) platform using 2D grapheneoxide (GO) and 0D plasmonic gold nanostar (GNS), with capability ofRaman enhancement factor (EF) in the range of ∼1010 via light–matter and matter–matter interactions. Thecurrent manuscript reveals huge Raman enhancement for heterostructurematerials occurring via both electromagnetic enhancement mechanismthough plasmonic GNS nanoparticle (EF ∼107) andchemical enhancement mechanism through 2D-GO material (EF ∼102). Finite-difference time-domain (FDTD) simulation data andexperimental investigation indicate that GNS allows light to be concentratedinto nanoscale “hotspots” formed on the heterostructuresurface, which significantly enhanced Raman efficiency via a plasmon–excitonlight coupling process. Notably, we have shown that mixed-dimensionalheterostructure-based SERS can be used for tracking of cancer-derivedexosomes from triple-negative breast cancer and HER2(+) breast cancerwith a limit of detection (LOD) of 3.8 × 102 exosomes/mLfor TNBC-derived exosomes and 4.4 × 102 exosomes/mLfor HER2(+) breast cancer-derived exosomes.

Plasmonic core-shell nanoparticles (CSNPs) have been extensively used as SERS active-substrates because their localized surface plasmonic resonance (LSPR) properties and thus the surface enhanced Raman scattering (SERS) activities can be regulated by changing the shell thickness. In this work, we selected Ag@MoS₂ CSNP with 40 nm radius of Ag as core and varied thickness of MoS₂ as shell to investigate the shell-dependent plasmonic behaviors including LSPR and SERS by using finite difference time domain (FDTD) simulations. The LSPR peak of Ag@MoS₂ CSNPs shows a broad red-shifting with an increasing shell thickness from 0 nm to 40 nm, giving rise to that the LSPR peak tunes from visible region (385 nm) to near infrared (NIR) region (1100 nm). The SERS activity of Ag@MoS₂ CSNP, represented by the enhancement of local electrical field (EM), can also be modulated by changing the shell thickness, and the optimal enhancement factor (EF) under 633 nm laser excitation is determined to be 3.54&times;10⁶ when the shell thickness is 4 nm. The wide-range LSPR tunability of Ag@MoS₂ CSNP provides enormous potential for NIR SERS application and enhanced photocatalytic activity

Using optical sensors to transform light-matter interaction into optical signal has become more and more popular. This is especially true for the fields that require ultrafast responsibility and remote sensing, such as environmental monitoring, food analysis and medical diagnosis. Among numerous optical sensors, plasmonic nanosensors are of great promise due to their spectral tunability and good adaptability to modern nanobiotechnologies. Localized surface plasmon resonance (LSPR) is the electromagnetic resonance of conducting electrons on metal surface, and it is very sensitive to the variation of environmental refractive index. The LSPR is considered as a useful sensing parameter that provides very good biochemical information. The SPR absorption peak also can be adjusted by changing the nano structure on the LSPR biological sensor chip. In this study, Finite-Difference Time- Domain (FDTD) was applied to simulate the LSPR absorption peak. Four model parameters were modified to study the LSPR sensing sensitivity: (a) the incident light wavelength, (b) the diameter of nanoparticle, (c) the spacing among nanoparticles, and (d) the height of nanoparticle. The simulation results show that 860nm is the best wavelength for the LSPR adsorption measurement. The optimal diameter of nanoparticle is 150nm, and the nanoparticle spacing is 90nm. Higher nanoparticle height provides higher sensitivity, but it also depends on the process capability. The FDTD simulation can be a useful tool to design a LSPR nanoparticle biosensor.

The Discrete Dipole Approximation (DDA) method and the Finite Difference Time Domain (FDTD) method are used to analyze silver nanospheres with different radius and the coupling of nanospheres array complementarily. DDA method is used for simulating the extinction spectra of single silver nanosphere and nanospheres array; and the coupling of two nanospheres and their surrounding electric field distribution are simulated by FDTD method. Through these results, we got some important conclusions of nanoparticles’ Localized Surface Plasmon Resonance (LSPR) phenomenon.

Understanding the relaxation and injection dynamics of hot electrons is crucial to utilizing them in photocatalytic applications. While most studies have focused on hot carrier dynamics at metal/semiconductor interfaces, we study the in situ dynamics of direct hot electron injection from metal to adsorbates. Here, we report a hot electron-driven hydrogen evolution reaction (HER) by exciting the localized surface plasmon resonance (LSPR) in Au grating photoelectrodes. In situ ultrafast transient absorption (TA) measurements show a depletion peak resulting from hot electrons. When the sample is immersed in solution under -1 V applied potential, the extracted electron-phonon interaction time decreases from 0.94 to 0.67 ps because of additional energy dissipation channels. The LSPR TA signal is redshifted with delay time because of charge transfer and subsequent change in the dielectric constant of nearby solution. Plateau-like photocurrent peaks appear when exciting a 266 nm linewidth grating with p-polarized (on resonance) light, accompanied by a similar profile in the measured absorptance. Double peaks in the photocurrent measurement are observed when irradiating a 300 nm linewidth grating. The enhancement factor (i.e., reaction rate) is 15.6× between p-polarized and s-polarized light for the 300 nm linewidth grating and 4.4× for the 266 nm linewidth grating. Finite-difference time domain (FDTD) simulations show two resonant modes for both grating structures, corresponding to dipolar LSPR modes at the metal/fused silica and metal/water interfaces. To our knowledge, this is the first work in which LSPR-induced hot electron-driven photochemistry and in situ photoexcited carrier dynamics are studied on the same plasmon resonance structure with and without adsorbates.

The design of surface-enhanced Raman spectroscopy (SERS) platforms based on the coupling between plasmonic nanostructures and stimuli-responsive polymers has attracted considerable interest over the past decades for the detection of a wide range of analytes, including pollutants and biological molecules. However, the SERS intensity of analytes trapped inside smart hybrid nanoplatforms is subject to important fluctuations because of the spatial and spectral variation of the plasmonic near-field enhancement (i.e., its dependence with the distance to the nanoparticle surface and with the localized surface plasmon resonance). Such fluctuations may impair interpretation and quantification in sensing devices. In this paper, we investigate the influence of the plasmonic near-field profile upon the Raman signal intensity of analytes trapped inside thermoresponsive polymer-coated gold nanoarrays. For this, well-defined plasmonic arrays (nanosquares and nanocylinders) were modified by poly(N-isopropylacrylamide) (PNIPAM) brushes using surface-initiated atom-transfer radical polymerization. Molecular probes were trapped inside these Au@PNIPAM nanostructures by simple physisorption or by covalent grafting at the end of PNIPAM brushes, using click chemistry. The SERS spectra of molecular probes were studied along various heating/cooling cycles, demonstrating a strong correlation between SERS intensities and near-field spectral profile of underlying nanoparticles, as confirmed by simulations based on the finite difference time domain method. Thermoresponsive plasmonic devices thus provide an ideal dynamic SERS platform to investigate the influence of the near-field plasmonic profile upon the SERS response of analytes.

In this paper we perform a detailed and systematic investigation of electromagnetic field localization and enhancement, at different excitation wavelengths in the 520–640 nm domain, near spherical gold nanoparticles (AuNSs) of different sizes, using Finite-Difference Time-Domain (FDTD) simulations. We provide clear evidence of the size-dependent local electromagnetic field distribution and plasmon-dependent near-field intensity at the surface of plain and DNA-mimicking shell-coated individual AuNSs. This represents a crucial aspect which needs to be taken into consideration in the optimization of platforms based on AuNSs for plasmon-enhanced spectroscopies. Our set of FDTD simulations reveal useful insights regarding the extent of the spectral red-shift of the maximum electromagnetic field enhancement position relative to the localized surface plasmon resonance (LSPR) band, alongside an interesting AuNS size-dependent field enhancement variation in the 520–640 nm excitation range. Finally, we correlate some of the main important theoretical findings from FDTD simulations with experimental data from Surface Enhanced Raman Spectroscopy (SERS) and Metal Enhanced Fluorescence (MEF) assays based on particular types of plain and shell-coated AuNSs.